Formal charge, octet rule

It will always be true that a nitrogen with four covalent bonds has a formal charge of + 1 (A nitrogen with four co valent bonds cannot have unshared pairs because of the octet rule)... 

Among structural formulas in which the octet rule IS satisfied for all atoms and one or more of these atoms bears a formal charge the most stable reso nance form is the one in which negative charge re sides on the most electronegative atom... 

Isomtriles are stable often naturally occumng compounds that contain a divalent carbon An example is axisonitnle 3 which can be isolated from a species of sponge and possesses anti malanal activity Write a resonance form for axisonitnle 3 that satisfies the octet rule Don t for get to include formal charges... 

The concept of formal charge has a much wider applicability than this short discussion might imply. In particular, it can be used to predict situations in which conventional Lewis structures, written in accordance with the octet rule, may be incorrect (Table 7.2). 

For such odd electron species (sometimes called free radicals) it is impossible to write Lewis structures in which each atom obeys the octet rule. In the NO molecule, the unpaired electron is put on the nitrogen atom, giving both atoms a formal charge of zero ... 

Using only Lewis structures that obey the octet rule, draw the Lewis structures and determine the formal charge on each atom in (a) CHsf (b) OCL (c) BF4". 

The structures with expanded octets have lower formal charges, (b) There is one Lewis structure that obeys the octet rule ... 

Once you learn to recognize this pattern (a lone pair next to a pi bond), you will be able to save time in calculating formal charges and determining if the octet rule is being violated. You will be able to push the arrows and draw the new resonance structure without thinking about it. 

Not only molecules with LLPCN > 4, but all molecules of the elements in period 3 and beyond in their higher valence states, including most of their numerous oxides, oxoacids, and related molecules such as SO3 and (H0)2S04 should be regarded as hypervalent if AO bonds are described as double bonds (1). However, Lewis did not regard these molecules as exceptions to the octet rule because he wrote the Lewis structures of these molecules with single bonds and the appropriate formal charges (2). 

Take the organic substance called acetamide, for instance, a compound related to proteins. We can write two valid electron-dot structures for acetamide, both of which fulfill the octet rule for the C, N, and O atoms. One of the two structures has no formal charges, while the other has formal charges on the O and N atoms. (Check for yourself that the formal charges are correct.)... 

A-3. Write a second Lewis structure that satisfies the octet rule for each of the species in Problem A-2, and determine the formal charge of each atom. Which of the Lewis structures for each species in this and Problem A-2 is more stable ... 

Figure 1.9 illustrates the construction of a Lewis structure for a more complex example, nitromethane. In this case there are a number of reasonable (according to what we know now) constitutional isomers. Therefore, we must be told the connectivity, or which atoms are bonded to which. Once the connectivity shown in Figure 1.9 is given, the best structure that satisfies the octet rule has some formal charges. This still represents a quite stable molecule. 

Estimate the stability of a Lewis structure by whether it satisfies the octet rule and by the number and the distribution of the formal charges in the structure. (Problems 1.19, 1.20, and 1.36)... 

Discuss the stability of each of these species based on the octet rule and formal charges ... 

Which of these two structures is correct Both of them satisfy the octet rule and neither has formal charges, so both are predicted to be of comparable stability. On the basis of what we have discussed so far, we cannot predict which is more stable. In fact, both of these compounds are quite stable and can be put in a bottle. But they are different compounds. Ethyl alcohol is the alcohol found in beverages. It is a liquid at room temperature. In contrast, dimethyl ether is a gas at room temperature and is quite poisonous. As was mentioned in Section 1.7, compounds such as these, with the same molecular formula but different arrangements of bonded atoms (different structures or different connectivities), are called constitutional isomers (or structural isomers). Constitutional isomerism is very common in organic compounds. This is another reason why it is necessary to show the structure of the compound under discussion rather than just the molecular formula. 

The relative stability of resonance structures can be judged by the same rules that were previously introduced to judge the stability of Lewis structures the octet rule, the number and location of formal charges, and the interactions between charges in the structure. 

Although it has formal charges, this structure has the octet rule satisfied at all of the atoms. It is the most stable of all of these resonance structures. 

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